Media captionIan Pannell's report contains some harrowing testimony on the trafficking of children

In the US, poverty, deprivation and exploitation draw thousands of its own children down into a dark underworld that offers few ways out.

It is a world few Americans are aware of. But tens of thousands of American children are thought to be sexually exploited every year.

It's believed that every night hundreds are sold for sex.

The FBI says child sex abuse is almost at an epidemic level, despite the agency rescuing 600 children last year.

"Trafficking" often conjures images of people from other countries being smuggled over land and across the sea and then forced to work against their will in foreign lands. People are trafficked into America from Mexico, Central and South America. But the vast majority of children bought and sold for sex every night in the United States are American kids.

We have heard from a number of women from the East coast to the Mid-west who have frighteningly similar and horrific stories. Neglected, abused, exploited and often ignored starting from a young age - sometimes even prosecuted by the very people who should have protected them.

A handful of good souls, the kindness of a few strangers and the good work of some law enforcement agencies and the FBI offer some relief to America's most vulnerable. But the stories we have heard suggest they are only scratching the surface of one of America's best-kept and darkest secrets.

When a choice is not a choice

In Minnesota, I met with former sex workers who had sought support through an advocacy group called Breaking Free. Half of the women in the group were under the age of 18 when they first were sold for sex. Many of the others were not much older than 18.

One woman says she was bought by her aunt at the age of 14.

"She gave my mom $900. Told me I was going shopping at the mall."

The aunt would bring her to drug dealers' houses, where she was raped and given drugs.

"She would leave me...and then [was] like 'You were messed up, you wanted to stay'," she recalls. She soon believed the abuse was her fault and her choice.

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Media captionGenesis: "All of this was normal," she says. "I'd seen it since I was 10, 11, 12 years old."

Another woman says she was 17 when she was kicked out of the house.

"I wanted to get high," she says, and turned to working as a prostitute. She later started using the classified adverts website Backpage.com to make more money to keep up with her addiction.

A third was 14 when she was kidnapped by "a guy I thought I liked". She didn't return home for two years.

Jenny Gaines, who leads the group discussion at Breaking Free, says many "manipulate and take advantage of underage girls".

One woman said of her abuser, "He knew I was 14, he had to know that I was underage," despite her attempts to pretend to be 18.

"When he actually found out how old I was it didn't stop him... he wanted me even more."

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Media captionJenny Gaines: Tricked into prostitution at 14

Fighting to stay out of 'the life'

A woman who was first trafficked at 14 says she is living in a shelter right now and is struggling to not return to prostitution.

"There's tricks' names still on my phone, I haven't even deleted them yet and I need to delete them," she says. "Because when I get down, when I'm feeling really yucky it's almost like I want to have that number there."

But she says she doesn't want to return to that life.

"It's just a big circle, you get high, you get tricked, you get the money and you just keep going around and going around, and you have to break off all of them to even be doing okay."

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The women at Breaking Free support each other while they discuss the difficulty of leaving sex work

It is an uphill battle.

"I just need the support and to believe in myself that I can make it. It's a funny spot I'm in."

Another woman says she hasn't been on Backpage for eight months.

"I'm not perfect. I'm just trying," she says. She finds it difficult to provide for her daughter without the money she made working as a prostitute.

"I stopped when I was 22 and had my first son," the woman says, detailing her "off and on" experience. She's been away for seven months, partially because she is pregnant with her fourth child.

She hopes attending Breaking Free will prevent her from returning.

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Gaines listens to a woman speak at Breaking Free

"I'm going to have a daughter," she says. "I don't want her to do like what I did."

Another woman likens it to an addiction.

"It's like I have this hole like whatever it is it's not enough, that fills it for me, my kids get what they want," she tells the group. "I don't ever have to ask nobody for nothing,"

Many of the women in the Breaking Free yearn for sense of normalcy.

"I just want my freedom back," one woman says. "I just want to look out for my kids, and live my life, live a normal life."

But for women who were sold for sex as children, abuse, drugs and sex work is normal.

One woman we spoke to in Minnesota was not at Breaking Free. She was on the streets, still working at five months pregnant.

She says was groomed from age 12 by a neighbour, who enticed her with a garage full of toys and games. He offered her money for topless photos.

"I see more and more younger girls out here now and it's really sad," she says.